1. Field of the Invention
The present invention relates to a thermoelectric conversion device and particularly to a thermoelectric conversion device including an electrode for electric field to thermoelectric conversion material.
2. Description of the Related Art
Conversion of thermal energy into electric energy using Seebeck effect on a substance is called thermal conversion, and a device capable of the thermal conversion is a thermal conversion device. A material used in the thermal conversion device is called a thermal conversion material. As an index for estimating a thermal conversion effect, a performance index of Z=S2σ/κ is used where S is a Seebeck coefficient, σ is an electric conductivity, and κ is a thermal conductivity.
There are known thermoelectric conversion materials such as (1) a material made of a compound of a semiconductor such as Bi—Te, Si—Ge, and Zn—Sb or a compound having a Skutterudite structure, (2) a material made of NaCoO2 representative of oxide, and (3) compounds having a half Heusler structure such as ZrNiSn.
However, the conventional materials listed above have limitations in the electric conductivity and the Seebeck coefficient. The performance index necessary for realizing the thermoelectric conversion device is defined by ZT (T is a temperature). Generally a ZT equal to or more than one is required, and a ZT equal to or more than two is required partially.
To solve this problem, JP 2009-117430 A discloses a thermoelectric device including a pair of a source electrode S and a drain electrode D for taking out an electromotive force according to a thermal gradient generated in a semiconductor A having a carrier density equal to or smaller than 1022/cm3 and for generating the thermal gradient in the semiconductor A by conducting a current, and a gate electrode G for applying an electric field in the vertical direction to a conduction direction of the current between the source electrode S and the drain electrode D. In such a configuration, when a voltage is applied to the gate electrode G, a carrier density on a surface of the semiconductor A just under the gate electrode G varies. When the gate voltage becomes equal to or greater than a predetermined value, carriers are two-dimensionally confined on the surface of the semiconductor A just under the gate electrode G, a quantum effect of which generates a huge thermal electric power. Accordingly, a power factor can be maximized because both the electric conductivity σ and an absolute value |S| of the Seebeck coefficient can be increased. However, in JP 2009-117430 A there is also limitation in increase in the electric conductivity because the semiconductor is used.